Agenda - 01-03-1989 - Laserfiche WebLink

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Mr. John M. Link, Jr. December 30, 1988 Page 2 "' As was discussed in our previous letter report to you of July 19, 1988, conceptual designs and project cost estimates presented in the Phase I Study were based upon conventional earthfill dam construction. The Town of Hillsborough has proposed construction of a roller compacted concrete (RCC) dam on Seven Mile Creek that would impound a far larger reservoir than was judged to be feasible in the Phase I Study. As we have previously stated, we firmly believe that a reservoir with a storage capacity' substantially in excess of 2.1 billion gallons and/or a safe yield in excess of 6 mgd should not be contemplated on Seven Mile Creek. The present report does not address the relative merits of alternative types of dam construction. However, preliminary costs have been prepared for earthfill and RCC dams at both sites. We must stress that these costs are very preliminary, especially those related to RCC dam construction. RESULTS The appendix contains reservoir area—storage relationships for the two sites that are based on the new topographic maps. According to these relationships, the reservoir water surface elevations at the Eno River and Seven Mile Creek sites must be raised, respectively, 4.5 feet and •4 feet above the previously indicated levels to achieve the same storage volumes and safe yields. A summary of revised data for alternative dams at the two sites is presented in Table 1 . This table includes data for reservoirs impounded by alternative earthfill and RCC dams that are comparable to those evaluated in the Phase I Study and our July 1988 letter report. The new normal and maximum water surface, elevations for the alternative reservoir projects are shown, respectively, on lines 2 and 3 of this table. The data on line 1 and lines 4 through 7 of this table are the same as in previous studies. This data shows that the reservoir on the Eno River site would have a higher overall yield than any of the Seven Mile Creek alternatives. Revised reservoir area requirements are shown on lines 8 and 9 of Table 1 , and the revised property acquisition requirements are shown in line 10. Land and buildings within the respective reservoir areas (lines 10a and 10c, respectively) fall within the following criteria, as defined in the Phase I Study: • Property, that is within 50 feet of the normal reservoir shoreline. • Property that would be inundated at the maximum flood pool elevation. Mitigation property (line 10b) has also been determined based on the Phase I Study assumptions. A special classification, termed "additional buildings in critical area" (line 10d), has been created for buildings that • are located outside of the defined reservoir limits but in such close proximity that they would be affected to some extent by the reservoir development. We have taken the conservative approach of including costs for such buildings in the respective project cost estimates, although the buildings themselves could be preserved. Y ,